1. Field of the Invention
The present invention relates to a current load driving circuit for driving a current load element and a method for driving the same. In particular, it relates to a current load device comprising current load elements and current load driving circuits arranged in a matrix, and a method for driving the same.
2. Description of the Prior Art
In recent years, a device having cells arranged in a matrix, each of the cells comprising a current load element that operates depending on a current passing therethrough and a current load driving circuit for driving the current load, has been developed.
For example, in many light emitting display devices with an organic EL (electroluminescence) device serving as the current load element, pixels each comprising the organic EL device and a drive circuit therefor are arranged in a matrix and driven according to the active matrix method. FIG. 37 is a schematic plan view of a display apparatus of such a light emitting display device. As shown in this drawing, on a display apparatus 1, there are formed a plurality of control lines CL extending in a row direction (the control lines are assigned consecutive numbers #1, #2, . . . , #(K−1), #K, #(K+1), . . . ) and a plurality of signal lines SL extending in a column direction (the signal lines are assigned consecutive numbers #1, #2, #(M−1), #M, #(M+1), . . . ). A pixel 2 is formed at an intersection of the control line CL and the signal line SL. This display device is driven as follows: the control lines CL are selected one by one; in synchronization with selection of one control line CL, the signal lines SL are supplied with brightness signals for pixels connected to the selected control line CL; in this state, the brightness signals are written to the pixels in the selected row; and the pixels continues the illumination according to the respective written signals until the control line is selected again.
A typical configuration of the pixel of the light emitting display device according to this method is shown in FIG. 38 (referred to as a first conventional example, hereinafter). As shown in FIG. 38, the signal line SL (#M), a power supply line VCC, a ground line GND and the control line CL (#K) pass through the pixel 2, and a light emitting device LED has an anode connected to the power supply line VCC and a cathode connected to the drain of a TFT (thin film transistor) Q, and the source of the TFT Q is connected to the ground line GND. A switch SW1 is connected between the gate of the TFT Q and the signal line SL and controlled by the control line CL. A capacitance element C is connected between the gate of the TFT Q and the ground line GND.
An operation of the pixel according to this first conventional example is as follows. When the control line CL is selected, the switch SW is turned on. At this time, a voltage enough to supply a current according to a current-brightness characteristic of the light emitting device LED is applied to the gate of the TFT Q through the signal line SL so as to cause the light emitting device LED to emit light with brightness at an intended gray-scale level. The gate voltage is maintained (retained) by the capacitance element C, even when the control line CL is deselected and the switch SW1 is turned off. This operation enables the light emitting device LED to maintain brightness at an expected gray-scale level.
The first conventional example has a disadvantage. That is, when there is un-unifomity in TFT's current/voltage characteristics, even if a same voltage is applied to gates, the light emitting devices are supplied with various currents. Consequently, the light emitting devices are not supplied with a current enough to provide an expected brightness, and thus, the quality of the display device is reduced. In particular, there is quite large deviation of current/voltage characteristics of poly-silicon TFTs, which are often used in display devices, so that the image quality thereof is significantly reduced.
To solve the problem, there has been implemented a method of supplying a current to a transistor in the pixel circuit through the signal line, converting the current into a voltage by the transistor and maintaining (retaining) the voltage.
FIG. 39 is a circuit diagram showing an arrangement of the pixel of the light emitting display device according to the method of supplying a current signal through the signal line, which is disclosed in Japanese Patent Laid-Open No. 11-282419 (referred to as a second conventional example, hereinafter). As shown in FIG. 39, a signal line SL (#M), a power supply line VCC, a ground line GND and a control line CL (#K) pass through a pixel 2. A light emitting device LED has an anode connected to the power supply line VCC and a cathode connected to the drain of a TFT Q1, and the source of the TFT Q1 is connected to the ground line GND. A switch SW1, which is controlled by the control line CL, is connected between the signal line SL and the drain of a TFT Q2, and the TFT Q2 has the gate and the drain short-circuited and the source connected to the ground line GND. A switch SW2, which is controlled by the control line CL, is connected between the gate of the TFT Q1 and the gate of the TFT Q2. In addition, a capacitance element C is connected between the gate of the TFT Q1 and the ground line GND.
An operation of the pixel according to this second conventional example is as follows. When the control line CL is selected, the switches SW1 and SW2 are turned on. At this time, a current according to a current-brightness characteristic of the light emitting device LED flows through the signal line SL to cause the light emitting device LED to emit light with a brightness at an intended gray-scale level. This current flows between the drain and source of the TFT Q2. However, since the gate and drain of the TFT Q2 are short-circuited, the gate voltage thereof is set at a value for passing the same current through the TFT Q2 in a saturation region, and the voltage is retained by the capacitance element C. The TFT Q1 and the TFT Q2 form a current mirror. Thus, if current/voltage characteristics of TFT Q1 are equal to those of the TFT Q2, a current, whose value is equal to that of the current flowing through TFTQ2 and the signal line SL, flows through the TFT Q1 and is supplied to the light emitting device LED. Then, even if the control line CL is deselected, the gate voltage of the TFT Q1 is maintained (retained) by the capacitance element C. Therefore, the TFT Q1 can supply the current to the light emitting device LED, and the light emitting device LED can maintain brightness at an expected gray-scale level.
FIG. 40 is a circuit diagram of one pixel of another light emitting display device according to the method of supplying a current required for light emission with an intended brightness through the signal line, which is disclosed in “Digest of IEDM” (1998), pp. 875–878 by R. M. A. Dawson et al. As shown in FIG. 40, a pixel 2 of this light emitting display device comprises a signal line SL (#M), a power supply line VCC, a ground line GND, a control line CL1 (#K) and a control line CL2 (#K) passing therethrough, four p-channel TFTs (p-TFT, hereinafter) Qp1 to Qp4, a light emitting device LED and a capacitance element C. The p-TFT Qp4 has the gate connected to the control line CL2, the source connected to the power supply line VCC and the drain connected to the source of the p-TFT Qp1. The drain of the p-TFT Qp1, as well as the drain of the p-TFT Qp3 having the gate connected to the control line CL1, is connected to an anode of the light emitting device LED. The source of the p-TFT Qp3 is connected to the gate of the p-TFT Qp1, and a cathode of the light emitting device LED is connected to the ground line GND. The p-TFT Qp2 has the gate connected to the control line CL1, the source connected to the signal line SL and the drain connected to the source of the p-TFT Qp1 and the drain of the p-TFT Qp4. In addition, the capacitance element C is connected between the gate and source of the p-TFT Qp1.
An operation of the pixel according to this third conventional example is as follows. If the pixel 2 is selected, the control line CL1 (#K1) enters into an “L” state, the control line CL2 (#K) enters into an “H” state, the p-TFT Qp2 and the p-TFT Qp3 are turned on, and the p-TFT Qp4 is turned off. Then, a current according to a current-brightness characteristic of the light emitting device LED flows through the signal line SL (#M) to cause the light emitting device LED to emit light with a brightness at an intended gray-scale level. This current is supplied to the light emitting device LED through the TFT Qp2 and TFT Qp1. At this time, the p-TFT Qp1 has the drain and the gate short-circuited via the drain and source of the p-TFT Qp3 and operates in the saturation state, the gate voltage of the p-TFT Qp1 is set at a value to provide the current, and the voltage is retained by the capacitance element C. When the selection of the control line shifts from the lines #K to the next, the control line CL1 (#K) enters into the “H” state, the control line CL2 (#K) enters into the “L” state, and the supply of the current from the signal line SL to the pixel is stopped. However, the p-TFT Qp4 is turned on, and the current flows through this transistor. In this case, the gate voltage of the p-TFT Qp1, when the current from the signal line SL flows through the p-TFT Qp1, is maintained (retained) by the capacitance element C. Therefore, the p-TFT Qp1 can supply the current to the light emitting device LED, and the light emitting device LED can maintain a brightness at an expected gray-scale level.
According to the first conventional example described above, the brightness depends on the voltage signal. However, there is quite large deviation of current/voltage characteristics of poly-silicon TFTs, and even if the same voltage is applied to the gates of TFTs, the light emitting devices are supplied with various currents, and thus, the brightness thereof varies. Therefore, there is a disadvantage that it is difficult to cause the light emitting device to emit light with an intended brightness, and the quality of the display device is reduced.
According to the second conventional example, a pair of transistors forming the current mirror are each constituted by a TFT. However, unlike with a crystalline silicon transistor, it is possible that the transistors of the pair have current/voltage characteristics which are significantly different from each other even when they are disposed close to each other. Therefore, a difference in current/voltage characteristics appears between the transistor for retaining (converting) the current and the transistor for supplying the current to the light emitting device, and thus, it becomes difficult to reproduce an intended brightness with high precision.
In the case of the third conventional example described above, if the organic EL or the like is used as the light emitting device, the light emitting device has a capacitance of the order of several pF in parallel therewith, and the capacitance constitutes a load on the driving TFT. Thus, when a pixel is to be selected, it takes a long time for the current value of the driving TFT to settle at a value for supplying an expected current to the light emitting device and for the voltages of the parts to settle in a state where the expected current is supplied to the light emitting device. Therefore, if the selection period is shortened to accommodate higher definition, the selection period will expire before the gate voltage of the p-TFT Qp1 settles at a value at which the current flowing through the signal line equals to the current the p-TFT Qp1 supplies to the light emitting device, and thus, the p-TFT Qp1 cannot supply an expected current to the light emitting device. Then, the light emitting device LED emits light with an unexpected brightness, and thus, the image quality is reduced. That is, the third conventional example has a disadvantage in that enhancing the definition reduces the image quality.